Method For Making A Layered Paper Or Board Web

ABSTRACT

A method for manufacturing a layered paper or board web, in which method thick stock (M) delivered to the paper machine is diluted with tail water and the diluted stock (M 1 ) is fractioned in order to produce two different stock fractions (A,B). The first stock fraction (A), with finer material, is conducted into the surface layer or layers of the web, and the second stock fraction (B), containing coarser material, is conducted into the middle layer or layers of the web. Before feeding into a headbox ( 15 ), the coarse stock fraction (B) is diluted with water whose consistency is substantially lower than the consistency of the water added into the thick stock (M) before fractionating. When water containing less solids than the normal tail water dilutes the coarse stock fraction, the stock fractions used for different web layers can better maintain their different properties, which improves the result of layering.

The invention relates to a method for making a layered paper or board web, in which method thick stock delivered to a paper machine is diluted with tail water and the diluted stock is fractionated to produce two different stock fractions of which, the first stock fraction, containing on an average finer material than the second stock fraction, is conducted into the surface layer or layers of the paper or board web, and the second stock fraction, containing on an average coarser material than the first stock fraction, is conducted into the middle layer or layers of the paper or board web.

A stock feed system for a multi-layer headbox is disclosed in the U.S. Pat. No. 5,746,889, in which different stock concepts are produced out of the same fresh stock to be fed into different layers of the multi-layer headbox. The stock feed system contains at least one screen for transferring long fibers out of the stock that is destined to form the surface layer of the paper into the stock that is destined to form a layer placed in the interior of the paper. The system may also contain another screen for transferring short fibers out of the stock that is destined to form the layer placed in the interior of the paper into the stock that is destined to form the surface layer of the paper.

Fractionating methods that apply centrifugal cleaning are disclosed in WO published applications Nos. 0 129 311 A1 and 0 222 947 A1, in which accept from the first centrifugal cleaning step is transferred into the surface layers of the web to be formed, and the accept from a second or lower step is transferred into the middle layers of the web to be formed. By fractionating the stock, lower fines contents and higher average fiber length can be achieved in the middle layers of the web than in the surface layers of the web. On the other hand, higher fines and filler contents can be achieved in the surface layers of the web than in the middle layers, which, among other things, improves the printing properties of the paper.

The accept fibers separated during the fractionating of the stock are on an average shorter, thinner and more pliable, and the accept contains more fines and filler than the reject. For simplicity's sake, the accept from fractionation will be referred to as the fine stock fraction and the reject from fractionation will be referred to as the coarse stock fraction.

It is typical for fractionating that the consistency of the accept is decreased and the consistency of the reject is increased compared to the consistency of the feed. Because of this, the reject often has to be diluted before it can be conducted into the headbox of the paper machine. In WO 0 222 947, the rejects from the fractionating stages are diluted with tail water taken from a wire pit before they are transferred into. the next stage. In WO 0 129 311, the same tail water is being used for diluting both the accept and the reject before they are fed into the different layers of the multi-layer headbox. When the coarse stock fraction intended for the middle layers of the web is diluted with “impure” tail water, i.e. containing plenty of fines and filler, at least some of the advantages obtained with fractionating are lost. Because of the tail water, the drainability of the reject can go down almost to the level of the accept and the filler content can go up to near the filler content of the accept.

The object of the invention is to provide an improved method for manufacturing a web from a stock by layering the different fractions thereof. Especially, the object is to maintain the different properties of the stock fractions produced by fractionating, whereupon the objectives set for the layering of the stock are easier to reach.

In order to obtain these and other objects that will become evident later, the method according to the invention is characterized by what is defined in the characterizing part of claim 1.

When the coarse stock fraction is diluted with water that contains less solids originating from the stock than normal tail water, the different properties of the stock fractions used for different layers of the paper or board web can be maintained better than before, which improves the result of layering in paper or board manufacturing.

Tail water is a filtrate removed from the web being formed on the wire section, which water is collected into a wire pit or the like and which contains fines and filler originating from the paper stock. A major part of the solids contained in the primary tail water is returned into the process by using tail water for diluting the thick stock in the short circulation of the paper machine.

Water suitable for diluting the coarse stock fraction is characterized by the fact that the consistency of the water is substantially lower than the consistency of the tail. water used for diluting the thick stock before fractionating. Preferably, the consistency of the diluting water is less than 60% of the consistency of the primary tail water.

Examples of possible water fractions suitable for diluting the reject are cloudy and clear filtrate from the recovery of fibers, water coming from suction flatboxes, separately collected paper machine spray water, water coming from the press section and tail water fractioned by clarification or some other method. One option is to dilute tail water with a substantially cleaner water fraction such that the consistency of water used for diluting the coarse stock fraction is substantially lower than the original consistency of the tail water.

In the following, the invention will be described with reference to the examples in the appended figures, but the intention is not to limit the invention to these only.

FIG. 1 shows a schematic view of manufacturing of layered paper using fractionating of the stock and a multi-layer headbox.

FIG. 2 shows manufacture of a multi-layer web using fractionating of the stock and two different web-forming units.

According to FIG. 1, thick stock M is delivered into the paper machine via a wire pit 10, where it is diluted with tail water originating from the wire section. The diluted stock Ml is conducted with a pump P₁ into a first centrifugal cleaning step 11 of a centrifugal cleaner equipment, where the stock is fractionated into two stock fractions A and B. Fractionating is performed so, that the first stock fraction will contain fibers that on an average are thinner, shorter and more pliable and also more fines and filler than the second stock fraction B. Because of this, the stock fractions are in the following referred to as the fine stock fraction A and the coarse stock fraction B.

The fine stock fraction A from fractionation is conducted as two component flows Al and A₂ through pumps P_(A1) and P_(A2) and machine screens 13 _(A1) and 13 _(A2) into two layers 14 _(A1) and 14 _(A2) of a multi-layer headbox 15 that are used to form the top and bottom layers of the manufactured paper or board web.

The coarse stock fraction B from fractionation is diluted and the diluted stock flow B₁ is conducted through a pump P₂ into a second centrifugal cleaning step 12 of the centrifugal cleaner equipment, where impurities are extracted from the stock. The accept B₂ from the centrifugal cleaning step 12 is conducted through a pump P_(B) and a machine screen 13 _(B) into a layer 14 _(B) of the multi-layer headbox that is used for forming the middle layer of the paper and board web.

As the fractionating increases the consistency of the coarse stock fraction B, the stock B must be diluted before conducting it into the headbox 15. Conventionally, tail water D₁ taken from the wire pit 10, containing substantial amount of fibers, fines and filler, is used for dilution. In the solution according to the invention, a water flow D₀, whose consistency is lower than the consistency of the tail water D₁ taken from the wire pit 10, is conducted into diluting the coarse stock fraction B. This water can, for example, be taken from fiber recovery screen, spray water recovery or from suction flatboxes. Also water from the wire pit 10 can be used as dilution water, provided that enough solids have been extracted from it in a separate process stage (not shown). Dilution water, which is substantially cleaner than the tail water, can also be obtained by diluting water from the wire pit with a cleaner water fraction.

FIG. 2 illustrates another solution according to the invention. There, the diluted stock M₁ is fractioned in the first centrifugal cleaning step 11 of the centrifugal cleaner equipment into two stock fractions A and B. The fine stock fraction A from fractionation is conducted into a headbox 16, which feeds the stock on a fourdrinier wire 18 for forming a first web W₁. The coarse stock fraction B from fractionation is diluted with a water flow D₀, whose consistency is lower than that of the tail water D₁ from the wire pit 10, and it is conducted into a second centrifugal cleaning step 12 of the centrifugal cleaner equipment. The cleaned stock B₂ is conducted through a pump P_(B) and a machine screen 13 _(B) into a headbox 17, which feeds the stock on a second fourdrinier wire 19 for forming a second web W₂. The second web W₂ is carried by the wire 19 on top of the first web W₁ on the wire 18 and the webs W₁ and W₂ are joined together to form a two-layered board web.

When manufacturing multi-layer web, there naturally can be more than two separate web forming units and the web forming units may comprise besides a fourdrinier wire also a gap former.

Instead of the above-described centrifugal cleaning device, fractionation can also be carried out in a manner known per se by using pressure screens. By adapting the fractionating techniques and the reject ratio, different types of properties can be obtained for the different layers of the web. There can naturally be more than one fractionating stage.

In the following the patent claims will be given, and the details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from the details given above for the sake of example only. 

1-10. (canceled)
 11. A method for manufacturing a layered paper or board web, comprising the steps of: delivering thick stock to a paper machine; diluting the thick stock with tail water of a consistency; fractionating the diluted thick stock to produce two different stock fractions comprised of a first stock fraction and a second stock fraction, wherein the first stock fraction contains on average finer material than the second stock fraction, and wherein the second stock fraction contains on average coarser material than the first stock fraction; and diluting the second stock fraction with dilution water having a consistency that is substantially lower than the consistency of the tail water added into the thick stock before fractionating, wherein the first stock fraction and the diluted second stock fraction are each fed into a headbox, and the first stock fraction is conducted into a surface layer or layers of the paper or board web, and the diluted second stock fraction is conducted into a middle layer or layers of the paper or board web.
 12. The method of claim 11, wherein the dilution water has a consistency which is at the most 60 percent of the consistency of the tail water used for diluting the thick stock before fractionating.
 13. The method of claim 11, wherein the dilution water is water from fiber recovery, suction flatboxes and/or a press section and/or separately collected fabric conditioning water and/or cleaned tail water.
 14. The method of claim 11, wherein the dilution water is tail water that has been diluted with a water type substantially cleaner than the tail water itself.
 15. The method of claim 11, further comprising the step of conducting the second stock fraction into the headbox through a cleaning device, wherein the second stock fraction is diluted before it is fed into said cleaning device.
 16. The method of claim 11, wherein the step of fractionating the diluted thick stock to produce two different stock fractions, comprises fractionating the diluted thick stock in centrifugal cleaners.
 17. The method of claim 11, wherein the step of fractionating the diluted thick stock to produce two different stock fractions, comprises fractionating the diluted thick stock by screens.
 18. The method of claim 11, wherein the step of fractionating the diluted thick stock to produce two different stock fractions, comprises fractionating the diluted thick stock in two or more stages.
 19. The method of claim 11, wherein the first stock fraction and the diluted second stock fraction are fed into the same headbox which is a multi-layer headbox, the first stock fraction being used for forming a surface layer of the paper or board web, and the second stock fraction being used for forming a middle layer of the paper or board web.
 20. The method of claim 11 wherein the first stock fraction is conducted into at least one headbox used for forming a surface layer of the multi-layer web, and the second stock fraction is conducted into at least one other headbox used for forming a middle layer of the multi-layer web.
 21. A method for manufacturing a layered paper or board web, comprising the steps of: delivering thick stock to a paper machine; diluting the thick stock with tail water of a consistency; fractionating the diluted thick stock to produce two different stock fractions comprised of a first stock fraction and a second stock fraction, wherein the first stock fraction contains on average finer material than the second stock fraction, and wherein the second stock fraction contains on average coarser material than the first stock fraction; feeding the first stock fraction into a first headbox and conducting the first stock fraction into a surface layer or layers of the paper or board web; taking dilution water from a recovery screen, or a spray water recovery, or suction flatboxes, or from a wire pit and extracting solids therefrom in a separate process stage, or from the wire pit and diluting with a cleaner water fraction, said dilution water having a consistency that is substantially lower than the consistency of the tail water added into the thick stock before fractionating; and diluting the second stock fraction with the dilution water, and then feeding the diluted second stock fraction into the first headbox or a second headbox and conducting the diluted second stock fraction into a middle layer or layers of the paper or board web.
 22. The method of claim 21, wherein the step of diluting the second stock fraction with dilution water comprises diluting the second stock fraction with water whose consistency is at the most 60 percent of the consistency of the tail water used for diluting the thick stock before fractionating.
 23. The method of claim 21, wherein the first stock fraction is conducted into at least one layer of a multi-layer headbox used for forming a surface layer of the paper or board web, and the second stock fraction is conducted into at least one other layer of the same multi-layer headbox used for forming a middle layer of the paper or board web. 